The use of optical detecting devices to determine the presence and/or size of particles is well known, and is shown, and such detecting devices have hertofore included lasers to provide the illuminating beam (see, for example, U.S. Pat. Nos. 3,406,289 and 4,571,079).
An optical surface scanning device, commonly referred to as a wafer scanner, has been heretofore utilized in semiconductor processing to identify surface adhering particulates. Semiconductor wafers are normally run through various manufacturing process steps (often as "virgin" wafer witness blanks) to determine the amount of contamination deposited on the wafer and assumed to be generated by the process. The process may be, for example, a photoresist coating process, a silicon oxide forming furnace operation, a photolithographic circuit masking process, or any of an array of others, but the wafer scanner has proved to be a useful tool to monitor any of them. Although independent techniques are also utilized to measure the microcontamination in various process fluids, the wafer scanner has nevertheless been utilized to provide monitoring where it directly effects the product, i.e., on the wafer itself.
As with any measuring tool, it is a fundamental requirement of apparatus measuring microcontamination to possess repeatability and accuracy. Repeatability or reproducibility must be inherent to achieve accuracy but places no guarantee of accuracy. Resolution implies an ability to separate measurement values of similar magnitude but again does not assure accuracy. Indeed, as with many types of instrumentation, the determination of performance with respect to resolution, repeatability, and accuracy can often be very difficult to assess and verify.
This proves to be the case with wafer scanners which appear to often generate an intolerable disparity of results when particles of known size were deposited on test wafer surfaces and analyzed. Considering that the contaminants are located in a thin plane (so that a shallow depth-of-field is required), the background light level is extremely low, and repetitive measurements are possible, it would appear that surface microcontamination measurements should be more easily accomplished than are aerosol measurements. However, aerosol instruments now known have far superior resolution than do now known wafer scanners.
A wafer scanner is, in reality, a fairly straightforward device, and three fundamental types of wafer scanners are generally known. In one, the wafer moves along a belt or other transport mechanism (usually stepper driven) and a laser beam is focused onto a small spot on the surface. The spot is dithered laterally by an oscillating mirror (or a rotating polygon), and the combination of the two motions generates a raster scan suitable for direct cathode ray tube display of coordinates illuminated on the wafer. The light reflected from the wafer is trapped or monitored for extinction, while the diffusely scattered light is analyzed for its bulk scattering (DC or low frequency changes in diffuse scattering) and point scattering events from particles or perhaps digs and scratches (AC or high frequency changes in diffuse scattering).
A second type of wafer scanner now generally known differs from the first above described type in that the wafer rotates on a chuck fixed to a transport stage, and light is incident and collected at fixed angles. The conversion of scanning polar coordinates to display cartesian coordinates can be accomplished easily in software or by electrical resolvers.
The third type of wafer scanner now known is the simplest optically. The wafer is totally illuminated by white light generated from a high intensity source such as an arc lamp. At an oblique angle, a vidicon (usually solid state) is positioned to view the wafer and particles appear directly on the vidicon output. This system is, however, the most complex in terms of analysis requiring post analysis of video data.
In all of the known wafer scanner systems, particles or defects are analyzed by measuring the amount of light scattered and compared to a calibration, or response, curve in a manner similar to most common aerosol or hydrosol counters.
Polystyrene latex microspheres (PSL) are the most desirable particle calibration material, although attempts have been made to generate defect patterns via microlithography to effect "calibration wafers". Particles cannot, however, now be simulated using pattern defects in any certain way, and the users and manufacturers of wafer scanners have found difficulty in using PSL particles as a preferred calibration material for a variety of reasons, including: PSL particles cannot be applied as a liquid suspension because no solvent is clean enough to evaporate without leaving a residue that could be confused with the PSL particles; application of dry PSL particles using standard aerosol nebulizing techniques require some other means of verifying that the particles would attach and "stick"; response sometimes appears to be a function of position; and the results generally appear nonmonotonic and thus ambiguous (i.e., smaller particles gave greater signals than larger ones). The last situation is one familiar to most researchers who have studied aerosol counters and most aerosol viewing geometries have to be tailored to achieve a monotonic response.
It is also the primary reason existing instruments have not adopted PSL as the calibration standard. Existing technology does not produce a monotonic calibration with uniform spherical particles.
As can be appreciated from the foregoing, an improved system and method could therefore be advantageously utilized for effecting analysis of the surface of an element to determine particle contamination and/or defects on or below the surface of a material.